1. Field of the Invention
The present invention relates to a trapping device used in, for example, an evacuation system for evacuating a vacuum chamber in a semiconductor manufacturing apparatus.
2. Description of the Related Art
A conventional evacuation system will be described with reference to FIG. 14. A vacuum chamber 10 is a processing chamber for processing semiconductor devices through processes such as etching or chemical vapor deposition (CVD). The processing chamber 10 is communicated with a vacuum pump 12 through a pipe 14. The vacuum pump 12 is used to raise the gas pressure of the spent gas from the processing chamber 10 to an atmospheric pressure, and in the past, oil-lubricated rotary pumps were common but modern pumps are mainly so-called dry types which do not use any liquid lubricant.
When the degree of vacuum required in the processing chamber 10 is higher than the performance capability of the dry pump 12, an ultra-high vacuum pump such as a turbo-molecular pump is provided upstream of the dry pump. A gas scrubber 20 is provided in a downstream location of the vacuum pump 12, and depending on the characteristics of the exhaust gases, those gases which cannot be exhausted to the atmosphere directly, such as those that are poisonous or explosive, are treated in this device by such processes as adsorption, decomposition and absorption, and only the harmless gases are exhausted into the atmosphere. The pipe 14 is provided with valves in desired locations.
In such conventional vacuum systems, if the reaction byproduct contains a substance having a high sublimation temperature, that substance will leave a solid residue in the process of pressure rise and, in some cases, precipitate in the vacuum pump to cause failure of the pump.
For example, when aluminum is subjected to an etching process, using typical process gases such as BCl.sub.3, Cl.sub.2, the exhaust gas from the processing chamber 10 will contain a reaction byproduct AlCl.sub.3 as well as residual gaseous BCl.sub.3 and Cl.sub.2.
Among these gases, AlCl.sub.3 does not precipitate on the intake side of the vacuum pump where it has a low partial pressure, but while it is being pressurized to raise the partial pressure, it precipitates inside the vacuum pump and the solid particles adhere to the inside wall of the vacuum pump. This phenomenon is a cause of pump failure. Similar problems are encountered when byproducts such as (NH.sub.4).sub.2 SiF.sub.6 and NH.sub.4 Cl are formed for producing SiN thin film by CVD.
Conventional remedial approaches include:
(1) The vacuum pump is heated to maintain the pump at a high temperature so that the exhaust gas would not precipitate solid particles inside the pump and is eliminated through the pump in a gaseous state.
(2) A water-cooled trap is provided in an upstream location (intake side) of the vacuum pump so as to precipitate potential particles before they enter the vacuum pump.
In the first approach, although it is effective in preventing precipitation inside the vacuum pump, precipitation can still occur in a gas scrubber disposed downstream of the vacuum pump, and the resulting blockages of the filtering layers presented a serious operational and maintenance problems. In the second approach, the system has to be shutdown for cleaning of the traps resulting that the system productivity is sacrificed.
Therefore, it is desirable to be able to trap potential particles in the spent exhaust gas and to regenerate the trap automatically without having to shut down the system. Such a system would assure long service life of the pump, provide protection for gas scrubbers and reduce time loss so that the operational reliability of the manufacturing system will be improved.
One of the important considerations in such an interchangeable trap system is that precipitated particles accumulate in the traps in proportion to the volume of the exhaust gas, and such residues often contain substances that are harmful when sublimated or are expensive and should be recycled. Therefore, it is necessary to provide the regeneration step of separating or removing the particles from the traps in order for reuse of the traps themselves, detoxification, safening or retrieval of precipitate materials.
Conventional trap regeneration processes include an on-site high temperature treatment to vaporize the precipitated particles or neutralization washing process after removing the trap. In the former case, for substances having a high sublimation temperature, it is necessary to elevate the trap temperature, and in some cases, the process of treatment itself poses a high level of danger. If high temperature is applied near to the trap, it creates a situation of having a high-temperature section side-by-side with a low-temperature section. and can lead to operational difficulties, least of which is wasted energy. Additionally, regeneration process by sublimation needs longer period than trapping itself which makes the whole system inefficient. Another problem is that a large storage facility is required for recycling the recovered gases.
On the other hand, trap regeneration process by washing requires many steps such as trap removal, transfer to washing facility and storage of removed traps, and such work is also inefficient. Another problem is that many spare traps must be readied.
It is therefore needed a trapping device in which regeneration process can be performed efficiently and quickly so that the exhausting vacuum pumps and the exhaust treatment device can be operated for longer periods and so that the facility and operating costs for such regeneration processes will be reduced.
Also, in the conventional evacuation system, unreacted feed gas and other process gases are retained in the exhaust gas, and one of the considerations in such a regeneration processes is to recover expensive feed substances, such as gaseous silane. However, the conventional evacuation system is design to introduce all the exhaust gases into the same exhaust treatment device for processing, it is difficult to provide a seperate optimized treatment to each gas. Here, silane is being used to represent monosilane, disilane, dichlorosilane and all other silane group gases of different sublimation temperatures.
Another consideration in developing a new trap treatment process is that the costs of operation and capital for treating all the gases in one treatment device are high because of the large volume of spent gas to be treated and the consequent large scale of the processing facility and complex processing circuits necessary. Therefore, a trapping device and method is desired capable of efficiently trapping and retrieving expensive material such as silane.